Problem 7 - Chemistry in 30 days

Question

A white salt is readily soluble in water and gives a colourless solution with a pH of about \(9 .\) The salt would be (a) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{COONa}\) (c) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (d) \(\mathrm{CaCO}_{3}\)

Step-by-Step Solution

Answer

The salt is \(\mathrm{CH}_{3}\mathrm{COONa}\) (sodium acetate), which forms a basic solution.

Step 1: Understanding the Question

We need to identify which among the given salts dissolves in water to form a solution with a pH around 9, which indicates a basic (alkaline) solution.

Step 2: Analyzing Each Option

1. **Option (a)**: \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) is an ammonium nitrate, which typically forms a neutral solution, not basic. 2. **Option (b)**: \(\mathrm{CH}_{3} \mathrm{COONa}\) is sodium acetate. When dissolved, sodium acetate hydrolyzes to form an acetate ion, which can accept H+ ions, making the solution basic. 3. **Option (c)**: \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\), ammonium acetate, typically forms a neutral solution because the effects of the acetate and ammonium cancel each other out. 4. **Option (d)**: \(\mathrm{CaCO}_{3}\) or calcium carbonate is not very soluble in water and mainly dissolves slightly to form calcium ions and carbonate ions, the latter of which can react with water to produce a slightly basic solution, but not as pronounced as a pH of 9.

Step 3: Focusing on the Correct Option

Given the requirement for a basic solution with a pH of about 9, Option (b), sodium acetate (\(\mathrm{CH}_{3} \mathrm{COONa}\)), is the correct choice. Sodium acetate hydrolyzes in water to form a basic solution due to the formation of acetate ions which increase the pH.

Key Concepts

pH Levels

The concept of pH is central to understanding how solutions can be acidic, neutral, or alkaline. **pH** is a scale used to measure how acidic or basic a solution is. It ranges from 0 to 14, with a pH of 7 being neutral.
A solution with a pH less than 7 is acidic, while one with a pH greater than 7 is basic, or alkaline.

• **pH 0 to 6:** Acidic. Examples include lemon juice and vinegar.
• **pH 7:** Neutral. Pure water is the most common example.
• **pH 8 to 14:** Basic (Alkaline). Baking soda and sea water have pH levels in this range.

In the exercise, the solution with a pH of about 9 is basic. This indicates the presence of substances that can remove hydrogen ions (H⁺) from the solution, such as hydroxide ions (OH⁻) resulting from substances like sodium acetate. Understanding pH helps us determine the nature of the solution, whether it's likely to be corrosive, safe for consumption, or needing neutralization.

Solution Chemistry

Solution chemistry explores how different substances dissolve, interact, and react in liquids to form solutions. When a solute dissolves in a solvent like water, it creates a homogeneous mixture called a solution. The properties of a solution are based on the nature of solute-solvent interactions.
Sodium acetate (\(\mathrm{CH}_{3} \mathrm{COONa}\)) is a great example, as when it dissolves in water, it dissociates into sodium (\(\mathrm{Na}^{+}\)) and acetate ions (\(\mathrm{CH}_3\mathrm{COO}^{-}\)).

• **Dissolution**: The process of a solute dissolving in a solvent. Efficient dissolution leads to transparent solutions.
• **Solubility**: Measures how well a solute can dissolve in a solvent at a given temperature. High solubility results in higher concentrations of the solute within the solution.
• **Ion formation**: Formation of ions like acetate can affect pH and solution behavior.

When sodium acetate dissolves, the acetate ions undergo hydrolysis, accepting hydrogen ions (H⁺) from the water and increasing the pH, thus creating a basic solution. Understanding these processes helps us predict the outcome of dissolving substances in various solvents and the resulting solution's properties.

Acids and Bases

Acids and bases are fundamental to many chemical reactions and processes. In general, acids are substances that can donate a proton (H⁺ ions) to other substances, and bases are substances that can accept protons.
Acids release H⁺ ions when dissolved in water, while bases produce hydroxide ions (OH⁻) or can accept H⁺, reducing their presence in the solution.

• **Acids**: Have pH less than 7, taste sour, and can react with metals to form hydrogen gas.
• **Bases**: Have pH greater than 7, feel slippery to the touch, and can neutralize acids by reacting with them.
• **Neutralization**: A reaction between an acid and a base, resulting in a more neutral pH.

Sodium acetate, the key focus of our exercise, acts as a weak base. By virtue of the acetate ion, which can accept H⁺ ions, it makes the solution less acidic and more basic, resulting in an elevated pH of around 9.
Understanding how substances like sodium acetate affect acidity and basicity in solutions helps us identify their roles in chemical processes and reactions.